The present invention relates to seat cushions and backrests and more specifically to cushions and backrests which support the human body without creating pressure points or areas of contact which support disproportionate weight distributions. Such disproportionate weight distributions often cause circulatory disfunction in the extremities after prolonged periods of immobility.
Since the erect position presents many anatomical disfunctions in the lower portions of the human anatomy, the sitting position creates even more problems of reduced circulatory efficiency due to restrictions in the normal flow of blood to and from the legs. The normal positions of the arteries, veins, and nerves are squeezed, pinched, and kinked by the displacement of muscle tissue and fat tissue when a person sits. These problems are accentuated due to the firmness of the underlying skeleton supporting the upper torso.
Specifically, the ischial tuberosities and the coccyx exert as much as 80% of the weight of the torso in an area confined to a triangle about five inches on a side with its apex to the rear. This force concentration or pressure accounts for "hitting bottom" or "bottoming out" of a person on a cushion after prolonged sitting or the premature complete compression of the cushion upon which the person is sitting. As the body is moved from an erect attitude in order to avoid discomfort to these anatomical portions of greatest weight support, the position and subsequent distribution of weight on the spinal column is changed, thus causing posterior movement of the vertebrae in the lumbar region. Such movement causes stretching of the deltoid muscles of the back, irregular pressure on the vertebral discs, emphasis and increased pressure on the coccyx, and the like. Stretch of the deltoid muscles reduces the supportive and strength capabilities of these muscles which causes further relaxation and posterior curvature of the spine. Such further relaxation and posterior curvature of the spine causes additional pressure on the anterior side of the discs yielding nervous stress and subsequent reduction of efficiency. Holding the deltoid muscles in their stretched position brings on eventual premature fatigue of the individual.
Since the arteries and nerves are located to the rear of the skeleton especially in the extremities (presumably to protect them from injury), it follows that the sitting attitude of a person causes constriction and squeezing of these arteries and nerves between the femur and the cushion substructure of the chair or seat being sat upon, especially on the popliteal region behind the knee-joint. Resulting circulatory disfunction and nerve pinching are especially experienced by people whose occupations require long periods of sitting, such as stenographers, technicians in laboratories, computer operators, motorists, telephone operators, confinees of wheelchairs, and the like.
The art is replete with solutions to the above-described problem. Exemplary patents disclosing chairs specially designed to alleviate the foregoing problems include U.S. Pat. Nos. 3,193,328, 3,380,410, and 3,261,037. Also, attempts have been made to specially design beds in order to make them more anatomically acceptable as in U.S. Pat. No. 3,885,258. Such prior art proposals for providing specially designed chairs, beds, and the like, usually involve structures which are extremely complicated and expensive to manufacture. For example, U.S. Pat. No. 4,161,045 describes a mattress having large number of relatively incompressible ribs inserted into the mattress. The formation of this mattress requires that the main part of the mattress be formed in two pieces, that channels be cut in both parts of the mattress to accommodate ribs, that the ribs be inserted into the channels and finally that the two parts of the mattress be secured together. Such a complicated assembly is time-consuming and expensive.
Some of the prior art proposals for supporting the human body without creating pressure points involve varying the density of the elastic material from which a cushion or mattress is formed in direct proportion to the pressure upon any particular portion of the cushion or mattress; a typical example of such prior art proposals is U.S. Pat. No. 3,751,111. We have now discovered that such proposals are incorrect and exacerbate rather than ameliorate the problem. Providing less elastic material at points of high pressure only increases the pressure at these points, thus placing even greater pressure upon the arteries and nerves and causing even greater circulatory disfunction and nerve pinching. It should be noted that this extra pressure concentration caused by putting the hardest parts of the cushion or mattress in contact with the parts of the body exerting maximum pressure is not described in the literature and was only discovered during our researches leading up to the instant invention.
A further problem with some of the prior art proposals for variable-density cushions, including the aforementioned U.S. Pat. No. 3,751,111, is that they propose to construct the composite cushion by placing preformed blocks of one type of elastic foam material (usually the higher density material, since it is normally the higher density material which is in the form of discontinuous inserts within a matrix of lower density material) in a mold and then foam the lower density material around the preformed blocks of higher density material, thus securing the blocks in the correct relative positions. Although this has not previously been described in the literature, we have found that when one type of foam is foamed in situ around a preformed block of a second type of foam, a chemical reaction takes place between the foam which is being formed and the pre-existing foam, and this chemical reaction leads to the formation of a skin of material along the surfaces where the two foams are in contact, this skin being considerably harder and less resilient then either of the foams from which it is derived. Where, as is inevitably the case at some points, this skin of hard, non-elastic material extends perpendicular to the flat surface of the cushion on which the user sits, the user receives the very uncomfortable sensation of sitting on a sharp edge of hard material, which is not only uncomfortable but which further exacerbates the problems with circulation and nerve pinching already mentioned.
It might be thought that the problems associated with the hard skin formed when one foam is foamed in situ around an insert of another foam could be avoided by foaming the first foam in contact with a solid former of a material to which the foam will not adhere, and then removing the solid former and replacing it with the desired insert of the second foam. However, our researches have shown that this also is an unsatisfactory procedure in many cases because when commercial polyurethane and some other foams are foamed in contact with a solid former a tough skin is formed adjacent the former and this tough skin again produces localized, almost non-deformable areas in the cushion which cause discomfort to the user and exacerbate the aforementioned circulatory disorders and nerve pinching.
According, there is a need for a cushion which will avoid the aforementioned problems associated with prior art cushions, and for a method for forming such a cushion. This invention provides such a cushion and method for forming the cushion.